drupal_get_schema_versions() is about getting all the different hook_update_N() functions. It's not directly related to hook_schema(), drupal_get_schema(), or Schema API.
While a hook_update_N() *can* change a module's schema, it doesn't necessarily. Some hook_update_N()s just clear caches, or perform housekeeping on module data.
Comments
Comment #8
catchMarking duplicate of #2124069: Convert schema.inc to the update.update_hook_registry service (UpdateHookRegistry) which has a patch.